Click "File > Open" from the WaveLab tool menu. Browse to the location of the WMA file, click it once, then click "Open" to import the file into Wavelab.
Click "File > Save Special" and choose "MP3" from the choices.
Choose the MP3 resolution from the listed choices. You can choose 128 kbps for standard mp3, or 256 or 320 kbps for high-resolution MP3.
Choose a folder to save the MP3 to from the file location field. When you click it, you also have the option to save a new folder.
Click "Save." WaveLab will create an MP3 from the original WMA file.